Global Distribution
Shaded areas on the map indicate the world-wide distribution of D. plexippus (the Monarch Butterfly). The stippled areas represent the range of the subspecies D. plexippus plexippus, and the striped areas the range of D. plexippus erippus.
Monarchs are native to North and South America, but spread throughout much of the world in the 1800's. They were first seen in Hawaii in the 1840's, and spread throughout the South Pacific in the 1850's-60's. In the early 1870's, the first monarchs were reported in Australia and New Zealand.
You may be wondering how monarchs traveled across the Pacific Ocean. You are not alone. Researchers are still unsure exactly how and why the monarch made its journey across the ocean. A few hypotheses have been formulated. It is possible that they were transported by the numerous ships that made the long voyage across the Pacific Ocean. Because larvae have to move around a lot to find suitable pupation sites, and since milkweed can be found around shipyards, it is possible that the larvae were transferred onto ships where they could travel a long distance before emerging. It is also possible that overwintering adult monarchs landed on ships and then were carried across during the winter. Even these hypotheses seem a little far-fetched, however. It is most likely that humans were involved in the process, but it is not known to what extent. Monarchs in North America can fly over 2,200 km during their migration, so it is possible that some made the journey on their own!
References
Ackery, P.R., and R.I. Vane-Wright. 1984. Milkweed Butterflies: Their Cladisitics and Biology. Ithaca, NY: Cornell University Press.
Gibbs, George. 1994. The Monarch Butterfly. Auckland, New Zealand: Reed Publishing.
R.I. Vane-Wright. 1993. The Columbus Hypothesis: An Explanation for the Dramatic 19th Century Range Expansion of the Monarch Butterfly. pp. 179-186 in Biology and Conservation of the Monarch Butterfly. Los Angeles: Natural History Museum of Los Angeles County.